I. Field of the Invention
The present invention relates to dilatation balloon catheters of the type employed in percutaneous transluminal angioplasty procedures, and more particularly to a method of molding such balloons to reduce their cone stiffness and thereby improve the maneuverability in smaller and more tortious passages of the vascular system.
II. Discussion of the Prior Art
Dilatation balloon catheters are well known for their utility in treating the build-up of plaque and other occlusions in blood vessels. Typically, a catheter is used to carry a dilatation balloon to a treatment site, where fluid under pressure is supplied to the balloon, to expand the balloon against a stenotic lesion.
The dilatation balloon is affixed to an elongated flexible tubular catheter proximate its distal end region. When the balloon is expanded, its working length, i.e., its medial section, exhibits a diameter substantially larger than that of the catheter body on which it is mounted. The proximal and distal shafts or stems of the balloon have diameters substantially equal to the diameter of the catheter body. Proximal and distal tapered sections, referred to herein as "cones", join the medial section to the proximal and distal shafts, respectively. Each cone diverges in the direction toward the medial section. Fusion bonds between the proximal and distal balloon shafts and the catheter form a fluid-tight seal to facilitate dilation of the balloon when a fluid under pressure is introduced into it, via an inflation port formed through the wall of the catheter and in fluid communication with the inflation lumen of the catheter.
Along with body tissue compatibility, primary attributes considered in the design and fabrication of dilation balloons are their strength and pliability. A higher hoop strength or burst pressure reduces the risk of accidental rupture of the balloon during dilation. Pliability refers to formability into different shapes, rather than elasticity. In particular, when delivered by the catheter, the dilatation balloon is evacuated, flattened and generally wrapped circumferentially about the catheter in its distal region. Thin, pliable dilatation balloon walls facilitate a tighter wrap that minimizes the combined diameter of the catheter and the balloon during delivery. Furthermore, pliable balloon walls enhance the catheter "trackability" in the distal region, i.e., the ability of the catheter to bend in conforming to the curvature in vascular passages through which it must be routed in reaching a particular treatment site.
One method of forming strong, pliable dilatation balloons of polyethylene terrathalate (PET) is disclosed in U.S. Pat. No. RE. 33,561 (Levy). A tubular parison of PET is heated at least to its second order transition temperature, then drawn to at least triple its original length to axially orient the tubing. The axially expanded tubing is then radially expanded within a heated mold to a diameter about triple the original diameter of the tubing. The form of the mold defines the aforementioned medial section, shafts and cones, and the resulting balloon has a burst pressure greater than 200 psi.
Such balloons generally have a gradient in wall thickness along the cones. In particular, larger dilatation balloons, e.g., 3.0-4.0 mm diameter (expanded) tend to have a wall thickness in the working length in the range of from 0.010 to 0.020 mm. Near the transition of the cones with the working length or medial section, the cones have approximately the same wall thickness. However, the wall thickness diverges in the direction away from the working length, until the wall thickness near the proximal and distal shafts is in the range of 0.025 to 0.040 mm near the associated shaft or stem.
The increased wall thickness near the stems does not contribute to balloon hoop strength, which is determined by the wall thickness along the balloon medial region. Thicker walls near the stems are found to reduce maneuverability of the balloon and catheter through a tortious path. Moreover, the dilatation balloon cannot be as tightly wrapped about the catheter shaft, meaning its delivery profile is larger and limiting the capacity of the catheter and balloon for treating occlusions in smaller blood vessels.
U.S. Pat. No. 4,963,133 (Noddin) discloses an alternative approach to forming a PET dilation balloon, in which a length of PET tubing comprising the parison is heated locally at opposite ends and subjected to axial drawing to form two "necked-down" portions, which eventually become the opposite ends of the completed balloon. The necked-down tubing is then simultaneously axially drawn and radially expanded with a gas. The degree to which the tubing ends had been necked-down is said to provide control over the ultimate wall thickness along the walls defining the cones. However, it is believed that the use of the Noddin method results in balloons exhibiting a comparatively low burst pressure.
Copending application Ser. No. 08/582,371, filed Jan. 11, 1996, U.S. Pat. No. 5,733,301 describes a method for reducing cone stiffness by using a laser to ablate and remove polymeric material from the cone areas after the balloon is blown. It is preferable that the desired result be obtained during the balloon molding operations obviating the need for additional post molding operations.
Therefore, it is an object of the present invention to provide a method for stretch blow molding dilatation balloon having a high burst pressure and hoop strength, but with reduced material mass in the balloon cones, thus reducing cone stiffness and improving the trackability, crossing profile, stenosis recross and balloon retrieval, via a guiding catheter.